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Earth's light show is a clue to finding habitable neighbors

Posted August 29, 2001; 02:21 p.m.

by rstevens

Viewed from a star in some other corner of the galaxy, Earth would be a speck, a faint blue dot hidden in the blazing light of our sun.

Would there be any hint of that speck's amazing diversity of life? According to a paper in the Aug. 30 issue of Nature, a savvy alien would find at least one important clue: an interesting flicker in the pale blue light.

While our neighbors Venus and Mars would reflect a fairly even glow, Earth would put on a little show. Earth's light would brighten and dim as it spins, because oceans, deserts, forests and clouds -- which are all too small to be seen from such a distance -- reflect varying amounts of sunlight. The variations, it turns out, are so strong and distinctive that a surprising amount of information could be taken from a simple ebb and flow of light.

Scientists at Princeton University and the Institute for Advanced Study conducted a detailed study of Earth's reflections not for insights into an alien's view of our home planet, but as a way for human scientists to learn about distant planets that may be like our own. They are participating in the early planning for a NASA mission known as the Terrestrial Planet Finder, a space probe that will scan the skies for planets hospitable to life.

"If you looked at our solar system from far away, and you looked at the terrestrial planets -- Mercury, Venus, Earth and Mars -- one of the quickest ways to see that Earth is unique is by looking at the light curve," said Ed Turner, professor of astrophysics and a co-author of the study. "Earth has by far the most complicated light curve."

Eric Ford, a graduate student, and Sara Seager, a member of the Institute for Advanced Study, developed the idea in collaboration with Turner.

The standard thinking in the field had been that most of the information about an Earth-like planet would come from spectral analysis, a static reading of the relative component of different colors within the light, rather than a reading of changes over time. Spectral analysis would reveal the presence of gasses such as water vapor, carbon dioxide and oxygen, in the planet's atmosphere.

Looking at the change in light over time does not replace spectral analysis, but it could greatly increase the amount of information scientists could learn, said Turner. It may indicate, for example, the presence of weather, oceans, ice or even plant life.

"It's just one more tool, one more approach to a very tough problem," said Turner.

Although the idea that a planet's light would vary seems straightforward, the three scientists had no idea whether that variation would be large or small or what it would look like. After all, there are precious few opportunities to look at Earth from afar, noted Turner. He and colleagues reached their conclusions by studying existing research on the light-scattering properties of everything from cornfields to ocean waves. They then invented computer models of Earth that incorporated the data. The results showed variations in light of up to 150 percent over the course of a day, with characteristic signatures for different terrestrial features such as deserts, forests and oceans.

Turner said he and colleagues will continue to refine the idea. One possible way to test their conclusions is to measure changes in how much light from Earth is reflected off the moon, a phenomenon known as earthshine. But the real test will be if and when someone finds the first Earth-like planet. That moment could come in the next decade or so. NASA is exploring several alternatives for a planet-finding mission that would launch in 2012 or beyond, and also is seeking plans for smaller projects that could be launched in just a few years.

With characteristic understatement, Turner noted that if Earth-like planets were found "they would presumably be objects of tremendous interest."